(1) Field of the Invention
The present invention relates to a self-excited system voltage resonance type switching power source supply which is used as a power source for an inverter system microwave oven and, in particular, relates to an invention to protect against an over load of an inverter circuit from the time of turning on until oscillation of the magnetron starts and against damage caused by applying an extraordinary high voltage to the inverter transformer secondary side parts.
(2) Description of the Prior Art
According to a control circuit of the conventional inverter system microwave oven, the over load from the time of turning on to the time of initiation of oscillation of the magnetron was avoided by such like high voltage of the transistor which works as a switching element. The time from when the magnetron turns on to the time of starting oscillation was 1 to 5 second, which changes largely due to a peripheral temperature and the dispersion of the magnetron itself. Further, since controlling of such time by a charging time of a capacitor and a time constant delay circuit by a timer IC has no relation with the oscillation characteristics of the magnetron, it was not possible to obtain secure operation.
The switching power supply for a microwave oven is a flyback system using a leakage transformer in which the connection between the primary side and the secondary side is poor, and in which the magnetron, which forms a load for inverter circuit, becomes almost a no-load condition until the oscillation starts.
In this no-load condition, electro-magnetic energy which is supplied to the leakage transformer at the time when the inverter circuit is in the ON state is fed back to the inverter circuit itself as a flyback power at the time of OFF state. As a result there arises an over load condition, thereby causing the breakage of the switching transistor and so on, or application of an extraordinary high voltage on the inverter transformer secondary parts, thereby breaking the secondary circuit.
According to the conventional art, it was needed to contain high voltage and large amount of current transistor, capacitor, transformer winding and so on in order to prevent the breakage due to the over load of the inverter circuit. Thus, there resulted an expensive circuit having high allowance and surplus design.
A first object of the present invention is thus to form the circuit based only on operation in the ordinary loaded condition by reducing the over load of the inverter circuit at the time of no-load condition until the magnetron starts its oscillation after it turns on.
A second object of the present invention is to provide an improved switching power supply having high reliability with small size and low cost.
Other objects and features of the present invention will be clarified by the following explanation.